Conventionally, metal contaminants mixed in powder or the like of industrial materials, food, or the like, are detected, for example, by an electromagnetic induction detecting technique. This metal contaminant detection technique utilizes a change in magnetic field as an object under inspection passes through an alternating magnetic field generated by an excitation coil. Detection of a change in magnetic field by the detection coil indicates the presence of metal contaminant in the object. The excitation coil used for such an electromagnetic induction detection technique is normally a coil without an iron core.
Metal contaminants in powder are, in some cases, of the order of 0.1 mm to 70 micrometers in size. Conventionally, the detectable size of metal contaminants mixed in powder of nonconductive materials, such as foodstuffs, pharmaceuticals, resins, is up to about 0.5 mm for an electromagnetic induction technique. As such, conventional techniques for detecting metal contaminants have a problem of being incapable of detecting fine metal fragments.
This problem is caused by the low detection sensitivity of conventional excitation coils and detection coils used to measure metal contaminants. That is, since alternating magnetic fields that can be produced by excitation coils are small, a small change in magnetic field resulting from the presence of fine metal fragments is also small. This makes it impossible for detection coils to detect the change in magnetic field.
The detection coils also have a problem of size that increases with increasing number of turns of winding required to improve their detection sensitivity. Excitation coils also have the problem of increased size required to increase the number of turns of winding to enhance the level of alternating magnetic field. These factors contribute to an increase in the overall size of sensors.
This increase in the size of detection coils triggers the following problem. For detection of fine metal fragments, it is better suited for a detection coil to have a small detection spot. With a large spot of a detection coil, a change in magnetic field caused by fine metal fragments would be drowned in noise. On the other hand, reducing a detection spot would require a reduction in the size of the detection coil, such that the level of alternating magnetic field generated by the excitation coil would not be sufficiently large to take measurement.
JP-A-2004-85439 describes a conventional technique for detecting millimeter-scale metal contaminants. This technique uses a permanent magnet disposed at the center of a detection coil to detect millimeter-scale metal contaminants. Also disclosed is an opposite configuration used in this technique, that is, a detection coil is disposed at the center of a permanent magnet.
In this technique, disposing the permanent magnet at the center of the detection coil or disposing the detection coil at the center of the permanent magnet causes a problem of an increased transverse dimension of the sensor as a whole. Further, disposing the permanent magnet at the center of the detection coil causes a problem of increased size of the detection spot of the detection coil. On the other hand, disposing the detection coil at the center of the permanent magnet causes a problem of weak magnetic field generated by the permanent magnet.
JP-A-2005-83889 discloses a technique for overcoming the limitations of electromagnetic induction detection methods. To detect smaller contaminants, this technique utilizes the phenomenon of generating Joule heat as eddy current is induced in metal contaminants. This technique uses an infrared camera to detect infrared rays emitted from the metal contaminants.